The following description is presented with reference to an image projector implemented with a reflective light modulator of a digital micromirror device (DMD) type but is applicable also to image projectors implemented with other types of reflective light modulators. Image projectors currently implemented with DMDs require that the projector housing or DMD-illuminating light beam-directing optics contained within the projector housing be tilted at a 45 degree angle relative to a support table on which the image projector rests. This is done to cause the illuminating light to impinge on the DMD from either above or below its light reflecting surface and thereby provide a correct orientation of the DMD relative to a projection screen on which an image can be viewed. Inclining the projector or its components causes the projector to occupy an undesirably tall space when it is in use. Currently available single DMD projectors are taller than 10 cm in their operating positions. Using a tilting mechanism to thin the profile to less than 10 cm requires a tilting mechanism that raises the operating height by a corresponding amount.
FIGS. 1A, 1B, 1C, and 1D are respective isometric, frontal, side elevation, and top plan views of such a prior art image projector. With reference to FIGS. 1A, 1B, 1C, and 1D, a prior art image projector 10 includes a high power lamp 12 positioned at the focus of an elliptical reflector 14 to produce a high intensity illumination beam characterized by a principal ray 16 that propagates through a rotating color wheel disk 18 of a color wheel assembly 20. Disk 18 includes at least three sectors, each tinted in a different one of three primary colors to provide a field sequential color image capability for image projector 10. The illumination beam propagates through an integrator tunnel 22 to create at its output end a uniform illumination pattern that lens elements 24, 26, and 28 image onto a DMD 30.
The illumination beam propagating from integrator tunnel 22 is directed by a mirror 32 that is inclined so that the illumination beam propagates upwardly at a 45 degree angle relative to the plane of the supporting table for image projector 10 and exits lens element 26 toward a prism assembly 40. Prism assembly 40 is composed of prism components 42 and 44 that are spaced apart by an air space interface 46. After reflection by mirror 32, principal ray 16 of the illumination beam strikes a surface of lens element 28.
An incident light beam derived from principal ray 16 propagates through prism component 42 and, by total internal reflection, reflects off of a surface 50 at air space interface 46 to form a reflected incident light beam. The reflected incident beam propagates through prism component 42 to strike DMD 30. DMD 30 in its "on" light reflecting state (on-state) reflects an imaging light beam propagating normal to the plane of DMD 30 through prism component 42 and, without total internal reflection, through air space interface 46 into prism 44 to exit through an exit face 60 of prism component 44. The imaging light beam that passes through exit face 60 is characterized by a principal ray 62 and propagates through a projection lens 64 to a projector screen (not shown) to display an image to a viewer. DMD 30 in its "off" light reflecting state (off-state) reflects light by total internal reflection off of a face 68 of prism component 44.
The angles of the faces and the shapes of prism components 42 and 44 are selected so that the incident light beam, reflected incident light beam, and imaging light beam propagating within prism assembly 40 are coplanar. The arrangement of the components of image projector 10 results in the upward inclination of prism assembly 40 and thereby dictates for a housing (not shown) of projector 10 a minimum height that is greater than a minimum height that would be possible with an uninclined prism assembly and principal rays 16 and 62 propagating along essentially the same vector.